1. Field of the invention
The present invention relates to a rotary hook assembly for sewing machines and more particularly, to a rotary hook assembly of a sewing machine where a hook point or hook point member thereof is changeable.
2. Description of prior art
FIG. 1 shows a perspective view of a horizontal axis rotating hook assembly 1 typical in the prior art. Reference is made to FIG. 1 in the following explanation of such prior art assembly. A rotating hook body 2 formed generally in the shape of a bottomed cylinder has an opening 2a which is cut in a portion of a side wall thereof and defines an integral hook point 3. Fixed on the outer periphery of the rotating hook body 2 is a gib 4 for preventing a bobbin case holder (not shown) installed in the rotating hook body 2 from detaching therefrom. The gib 4 is provided with a gib claw 5 at a position opposing the hook point 3. In such known rotating hook assembly 1 constructed as described above, undesirable contact and rubbing of a needle against the hook point 3 during a sewing operation causes the needle and the hook point 3 to wear. Wear of the hook point 3 may prevent it from taking up needle thread loops, thereby resulting in missed stitches.
Also, collisions between the hook point 3 and the needle sometimes cause burr-like edges to form on the hook point 3, thus resulting in the disadvantage that the thread may be damaged by contact with such rising edges.
Moreover, since the hook point 3 is formed integrally with the rotating hook body 2, even when the gib 4 is removed from the rotating hook body 2, a space W1 between the hook point 3 which is one end of the opening 2a of the rotating hook body 2 and the opposite end of the opening is very small. Therefore, formation of the hook point 3 requires machining of a high-hardness material in a narrow space with high precision, resulting in poor machinability and problems in productivity.
As described above, the complete rotating hook assembly of a sewing machine of the prior art is not capable of good sewing performance for a long period of time in cases such as sewing thick fabric and sewing while automatically moving an object to be sewn such as cloth, and has poor productivity.